Conventionally, a CCD type image pickup apparatus using a CCD as its solid image pickup element is widely used as a solid image pickup apparatus using a solid image pickup element.
Hereinafter, a conventional solid image pickup apparatus using a CCD as its solid image pickup element will be explained (see JP2003-31789A).
FIG. 10 is a plan view showing the structure of a conventional solid image pickup apparatus. Furthermore, FIGS. 11A to 11C are cross-sectional views showing the structure of the conventional solid image pickup apparatus; FIG. 11A is an A-A′ cross-sectional view of FIG. 10, FIG. 11B is a B-B′ cross-sectional view of FIG. 10 and FIG. 11C is a C-C′ cross-sectional view of main components of FIG. 10.
In FIG. 10 and FIGS. 11A to 11C, reference numeral 21 denotes a semiconductor substrate, 22, 22a denote diffused layers selectively formed on the surface of the semiconductor substrate 21, 23 denotes an oxide insulating film formed over the entire surface of the semiconductor substrate 21 including the diffused layer 22, 24 denotes a first gate electrode, 25 denotes a second gate electrode, 26 denotes a light-shielding film, 27 denotes transparent resin or color resin and 28 denotes a lens. Note that since the bottom surface of each lens 28 is flattened, variations from one pixel to another are suppressed.
The operation of the solid image pickup apparatus having such a structure will be explained below. Light from an object is condensed by the lens 28, passes through the transparent (or color) resin 27 and only a light component entering a light-receiving area of the diffused layer 22 corresponding to an opening 26a of the light-shielding film 26 is photoelectrically converted. After the photoelectric conversion, the signal charge moves to a transfer area by applying a voltage to a second gate electrode 25. After the signal charge is moved to the transfer area, the signal charge is sequentially transferred through the interior of the transfer area diffused layer 22a by applying a voltage to the adjacent first gate electrode 24 and then further applying a sequential voltage to the adjacent second gate electrode 25, and finally output from this apparatus as an electric signal having a waveform corresponding to an intensity variation of the light from the object.
However, in the above described conventional solid image pickup apparatus, as shown in the A-A′ cross-sectional view of FIG. 11A, since the gate electrode is formed in a stack structure with two polysilicon layers of the first gate electrode 24 and the second gate electrode 25, the stack of the two layers of the gate electrodes 24, 25 exists in an area adjacent to the opening 26a of the light-shielding film 26 which constitutes the light-receiving area of the diffused layer 22.
For this reason, the light-shielding film 26 is elevated by an amount corresponding to the film thickness of one layer of the gate electrode in its stacked part, an amount of blocked light condensed by the lens 28, that is, “shading” increases, producing a problem that sensitivity decreases noticeably in finer pixels in particular.
The present invention is intended to solve the above described conventional problem and provides a method of manufacturing a solid image pickup apparatus capable of improving pixel sensitivity by improving the light condensing efficiency of a lens also for finer pixels and stably obtaining high sensitivity pixels.